I starts out as "Hear, O Israel, the LORD our God, the LORD is One". It's a Mezuzah, A Jewish charm you put on your door frame. depending on how you interpret the practice it goes on either your front door, or every door frame in the house. It's generally thought to ward off evil and keep the home safe.

I could only add one thing to ICryAtNight already comprehensive post: acording to legend, diagonal placment of mezuzah is result of unresolved dispute between two honorable rabies. Because they couldn't agree if mezuzah should be placed verticaly or horizontaly, to honor both of them, people started placing it diagonally.